The present invention relates to release films used for adhesive films, and particularly release films used for anisotropic conductive adhesive films.
Adhesive films have been widely used to bond a semiconductor chip to a wiring board.
Reference 110 in FIG. 5(a) represents an adhesive film of the prior art. Adhesive film 110 comprises a release film 111 and an adhesive layer 118 adhered to the surface of release film 111.
Release film 111 comprises a substrate 112 and a release agent layer 115 formed on the surface of substrate 112, and release agent layer 115 is provided on the side of release film 111 adhered to adhesive layer 118.
Reference 102 in FIG. 5(a) represents a wiring board. Wiring board 102 comprises a base film 103 and metal lines 104 provided on the surface of base film 103. For connecting this wiring board 102 to a semiconductor chip described below, the side of adhesive film 110 having adhesive layer 118 is first pressed against the side of wiring board 102 having metal lines 104 (FIG. 5(b)).
Generally, release agent layer 115 comprises silicone oil, and the adhesive force between silicone oil (release agent layer) and adhesive layer 118 is lower than the adhesive force between silicone oil and substrate 112 and also lower than the adhesive force between adhesive layer 118 and wiring board 102, so that release film 111 is separated from adhesive layer 118 and adhesive layer 118 is left on wiring board 102 when release film 111 is separated in the state shown in FIG. 5(b) (FIG. 5(c)).
Reference 105 in FIG. 5(d) represents a semiconductor chip comprising a chip body 106 and bump-like connection terminals 107 formed on one side of chip body 106. After the side of semiconductor chip 105 having connection terminals 107 is opposed to the side of wiring board 102 having metal lines 104 and the alignment is adjusted, semiconductor chip 105 is pressed against adhesive layer 118 on wiring board 102 and semiconductor chip 105 is heated under pressure in this state so that connection terminals 107 on semiconductor chip 105 come into contact with the surfaces of metal lines 104 on wiring board 102. Adhesive layer 118 develops adhesiveness by heating to mechanically connect semiconductor chip 105 to wiring board 102, and an electric device 101 shown in FIG. 5(e) is obtained.
When release film 111 having release agent layer 115 is used to form adhesive film 110 in this manner, release film 111 can be easily separated from adhesive layer 118 to connect wiring board 102 and semiconductor chip 105.
However, the release agent consisting of silicone oil has low adhesion to substrate 112, so that release agent layer 115 may be partially removed during preparation of adhesive film 110 or release agent layer 115 may be partially transferred to adhesive layer 118 during separation of release film 111 from adhesive layer 118.
When release agent layer 115 is removed from release film 111, adhesive layer 118 and substrate 112 come into direct contact with each other in the areas lacking the release agent layer and the adhesive force to adhesive layer 118 increases in those areas making it difficult to separate adhesive layer 118 from release film 111.
When release agent layer 115 transferred to the surface of adhesive layer 118 remains, the adhesive force between adhesive layer 118 and semiconductor chip 105 decreases to deteriorate the reliability of the resulting electric device 101.
As described in JPA 5-154857, it is known to use a releasable substrate such as a fluororesin film as a release film without forming a release agent layer.
However, fluororesin films are normally prepared by stretching and elongating the film during the preparation process so that they tend to vary in thickness, which compromises the precision of the width of the resulting films or the precision of the thickness of the adhesive films formed therewith.
When an adhesive layer is directly provided on the surface of a substrate without release agent, the adhesive force between the adhesive layer and the substrate (release film) directly depends on the surface roughness on the surface of the substrate. Therefore, any variation in surface roughness on the surface of the substrate causes variation in the adhesive force between the adhesive layer and the substrate.
In one aspect, the present invention relates to a release film comprising a substrate and a release agent layer formed on the surface of the substrate wherein the substrate has a surface roughness of 3 xcexcm or less on the side having the release agent layer, and the release agent layer is based on a fluorine compound.
In one embodiment, the present invention provides the release film wherein the substrate has a surface roughness of 1 xcexcm or more and 3 xcexcm or less on the side having the release agent layer.
In another embodiment, the present invention provides the release film wherein the release agent layer is formed by applying a release agent based on the fluorine compound in an amount of 0.01 g or more and 5 g or less per 1 m2 on the surface of the substrate.
In another embodiment, the present invention provides the release film wherein the contact angle of the release agent layer to water is between about 100xc2x0 and about 140xc2x0 and the contact angle of the release agent layer to silicone oil is between about 30xc2x0 and about 50xc2x0.
In one embodiment, the present invention provides the release film wherein the fluorine compound comprises at least one compound selected from the group consisting of C6H4(CF3)2, C8F16O and C8F18.
In one embodiment, the present invention provides the release film wherein the substrate has a thickness of between about 12.5 xcexcm and about 100 xcexcm.
In one embodiment, the present invention provides the release film wherein the substrate comprises a polyester resin formed in a film and a filler dispersed in the polyester resin.
In one embodiment, the present invention provides an adhesive film comprising a release film and an adhesive layer wherein the release film has a substrate and a release agent layer formed on the surface of the substrate, the substrate having a surface roughness of 3 xcexcm or less on the side having the release agent layer, and the release agent layer is based on a fluorine compound and the adhesive layer is formed on the surface of the release agent layer.
In one embodiment, the present invention provides the adhesive film wherein the adhesive layer includes a thermosetting resin.
In one embodiment, the present invention provides the adhesive film wherein the thermosetting resin is based on an epoxy resin.
As used herein, the surface roughness means the center line average roughness obtained from the height in direction Z measured by focusing on the surface of an object (substrate) using a laser microscope interferometer (eg, available from Lasertec Corp. sold under the name xe2x80x9cLaser Microscope ILM21xe2x80x9d).
In release films of particular embodiments in accordance with the present invention, the surface roughness of the substrate is 3 xcexcm or less. When a release agent layer is formed on the surface of such a substrate and an adhesive layer is formed on the surface of the release agent layer, the surface roughness on the side of the adhesive layer adhered to the release agent layer becomes 2 xcexcm or less.
If the surface roughness of the adhesive layer is as small as 2 xcexcm or less, excessive air does not enter between the object and the adhesive layer and any bubbles are not formed between the adhesive layer and the object when the side of the adhesive layer separated from the release film is applied on the object.
If the surface roughness of the substrate is greater than about 1 xcexcm, the adhesive layer is not separated from the release film when the adhesive film is slit because irregularities on the surface of the release film produces a moderate frictional force between the release film and the adhesive layer.
Irregularities (roughness) can be formed on the surface of the substrate by adding a filler in the substrate or roughing the surface of the substrate by corona discharge or abrasion or other means.
When a filler is added to the substrate, the surface roughness of the substrate can be controlled by modifying the particle diameter (average particle diameter) or the amount of the filler to be added.